1. Field of the Invention
This invention relates to a lift cam mechanism for an electronic parts-mounting apparatus, which lifts and lowers a mounting head which is moved about a vertical shaft to pick up and mount electronic parts.
2. Prior Art
Conventionally, an electronic parts-mounting apparatus of this kind has been proposed e.g. by Japanese Laid-Open Patent Publication (Kokai) No. 5-175695, in which an index unit, which forms a main part of a driving system of the apparatus, intermittently rotates a rotary table formed in a manner suspended from the index unit to thereby bring each mounting head arranged on the outer periphery of the rotary table to a suction station and a mounting station, while each mounting head is lifted and lowered to pick up an electronic part at the suction station by vacuum and mount the same on a circuit board at the mounting station. Each electronic part is picked up by vacuum from a pick-up point at the suction station and mounted at a mounting point at the mounting station. The pick-up point and the mounting point are different in level. Therefore, apart from being lifted and lowered directly for picking up and mounting electronic parts, the mounting head is progressively lifted and lowered by the lift cam mechanism comprised of a ribbed cylindrical cam disposed on the index unit side and cam followers fixed to the mounting head, as the mounting head is moved about a vertical shaft by intermittent rotation of the rotary table.
More specifically, a pair of upper and lower cam followers (roller followers) engage with a rib portion of the ribbed cylindrical cam such that the rib portion is interposed between the upper and lower cam followers in a manner rollable thereon, and the mounting head is suspended from a holder holding the cam followers, by way of a suspension member. The upper one of the cam followers is directly and rotatably fitted through the holder, whereas the lower one is rotatably fitted through a distal end of a swing arm pivotally or swingably mounted in the holder and at the same time urged toward the rib portion of the ribbed cylindrical cam by a coil spring interposed between and the swing arm and the holder. The coil spring ensures the reliable operation of the lift cam mechanism, and accommodates variations in thickness among individual rib portions formed on respective ribbed cylindrical cams and changes in thickness at a sloped portion of the rib portion of a particular cylindrical cam to thereby enable the cam followers to smoothly roll on the rib portion of the ribbed cylindrical cam.
The lift cam mechanism of the proposed electronic parts-mounting apparatus described above, however, suffers from the problem that a swinging stroke of the swing arm is very small and hence lubricant at a support shaft of the swing arm is liable to become short (since lubricant is difficult to spread over the whole peripheral surface of the support shaft), which often results in degraded durability of the swing arm and faulty following operation of the cam followers. Further, if there remain burrs at a portion of the swing arm pivotally fitted on the support shaft, the swing arm swings to an excessive extent, which can also cause the faulty following operation of the cam followers. As the apparatus operates at a higher speed, the faulty following operation of the cam followers comes to act on the mounting head as a shock, causing displacement or deviation in position of an electronic part picked up by the mounting head.
It is the object of the invention to provide a lift cam mechanism for an electronic parts-mounting apparatus, which is simple in construction and improved in durability, and capable of preventing the faulty following operation of cam followers following a cylindrical cam.
To attain the above object, the present invention provides a lift cam mechanism for an electronic parts-mounting apparatus including a vertical shaft and at least one mounting head arranged around the vertical shaft, the lift cam mechanism operating to lift and lower an associated one of the at least one mounting head as each mounting head is moved about the vertical shaft.
The lift cam mechanism according to the invention is characterized by comprising:
a cylindrical cam concentrically disposed with respect to the vertical shaft, the cylindrical cam having a protruding portion formed on a peripheral surface thereof in a manner such that the protruding portion has a generally annular shape and provides upper and lower cam surfaces;
a pair of upper and lower cam followers arranged respectively on the upper and lower cam surfaces of the protruding portion in a manner such that the pair of upper and lower cam followers sandwich the protruding portion therebetween in a manner rollable thereon;
a support member having one end and another end, the one end of the support member having one of the pair of upper and lower cam followers fitted therethrough and the another end of the support member having the associated one of the at least one mounting head connected thereto; and
a leaf spring having one end and another end, the one end of the leaf spring being fixed to the support member and the another end of the leaf spring having another of the pair of upper and lower cam followers fitted therethrough, for urging the another of the pair of upper and lower cam followers toward the protruding portion of the cylindrical cam.
According to this lift cam mechanism, the leaf spring urges one of the pair of upper and lower cam followers toward the cylindrical cam. Therefore, the one cam follower can be directly mounted through the leaf spring. That is, component parts of the lift cam mechanism conventionally formed by respective two separate members, i.e. a swing arm and a coil spring, can be formed by a single leaf spring, which makes it possible to eliminate a pivotally fixed portion of the swing arm for pivotal movement thereof. As a result, the pivotally fixed portion, which presents a problem of durability of the lift cam mechanism can be eliminated, and the loss of following operation occurring between component parts of the driving-force transmitting system of the cam mechanism can be minimized since the number of these component parts can be reduced.
Preferably, the leaf spring is integrally formed with the support member.
According to this construction, the number of the component parts of the lift cam mechanism can be further reduced and the leaf spring can be easily formed.
More preferably, the support member and the leaf spring are formed by a single plate member opposed to the protruding portion of the cylindrical cam, the support member being a thick portion of the plate member, while the leaf spring being a tongue-shaped portion of the plate member, which is formed by cutting a U-shaped cutout through the plate member and has a tip-side portion formed with a fitting portion through which the another of the pair of upper and lower cam followers is fitted and a root-side portion being processed by grinding to a reduced thickness to form a spring portion.
According to this preferred embodiment, the leaf spring can be integrally formed with the support member from a single plate member which is disposed opposed to the protruding portion of the cylindrical cam. Therefore, the leaf spring can be easily formed without bending a fitting portion through which a cam follower is mounted.
Further preferably, the root-side portion of the leaf spring has a central portion formed with a slot.
According to this preferred embodiment, it is possible to easily set a spring coefficient of the spring portion of the leaf spring.
Further preferably, the one end of the support member is formed with a through hole through which the one of the pair of upper and lower cam followers is fitted, the fitting portion of the leaf spring through which the another of the pair of upper and lower cam followers is fitted being a through hole.
Further preferably, the one of the pair of upper and lower cam followers is an upper cam follower placed on an upper one of the upper and lower cam surfaces of the protruding portion of the cylindrical cam.
Preferably, the electronic parts-mounting apparatus includes a support table, an index unit mounted on the support table and having an output shaft, a rotary table fixed to the vertical shaft, the vertical shaft being integrally formed with the output shaft of the index unit, the cylindrical cam being fixed to the support table.
More preferably, the lift cam mechanism includes at least one bracket vertically movably mounted on the rotary table, the at least one bracket having the at least one mounting head respectively fixed thereto and each connecting the support member to the associated one of the at least one mounting head.